The gloss of, e.g., paper products such as LWC-paper has been measured with a large number of different methods over a long period of time. The majority of these methods, and primarily the normative methods, are concentrated on mean gloss levels over a large surface area. Since the mean gloss level is not the only parameter contributing to the subjective impression of gloss quality, there is a need of an evaluation method that conforms better to the human evaluation of gloss quality, which includes the perception of gloss variation.
When studying the phenomena of gloss, particularly of printed paper, it has been found that the main caracteristics of the gloss variation can be described if the surface of the paper is approximated by a model surface comprising small flat surface elements, gloss facets, wherein gloss variation is determined on the basis of the orientation of the individual facets with respect to the plane of the paper and the reflected intensity of the individual facets. However, no method and apparatus that can be used in practice have been proposed for determining gloss variations based on this gloss facet approximation.
One object of the present invention is to provide a method and apparatus of the aforesaid kind which can be used effectively to carry out a multiple of different types of gloss variation evaluations on a sample surface in accordance with the principles of the aforesaid gloss facet approximation.
The object is achieved with the features set forth in the following claims.